JAMMA is a standard connector used on many arcade boards to simplify
conversion of cabinets from one game to another. The majority
of games use a subset of this pinout. Some games (Street Fighter,
Mortal Kombat, etc) which need extra buttons have extra connectors
for these additional controls. The JAMMA connector is a 3.96mm
pitch-edge connector (male on game board).

1 Non-standard buttons, seen on some games, but not part of the JAMMA standard.2 In the 2010's, Chinese JAMMA boards started mapping the grounds on pins 'e' and '27' to INPUTs, to avoid using a KICK harness.
** JAMMA Trackball pinout differences, used for games like Capcom Bowling.